Phosphor, and radiation image storage panel

ABSTRACT

A bismuth activated alkali metal halide phosphor having the formula (I): 
     
         M.sup.I X:xBi                                              (I) 
    
     in which M I  is at least one alkali metal selected from the group consisting of Rb and Cs; X is at least one halogen selected from the group consisting of Cl, Br and I; and x is a number satisfying the condition of 0&lt;x≦0.2. A process for the preparation of said phosphor, a radiation image recording and reproducing method utilizing said phosphor, and a radiation image storage panel employing said phosphor are also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a novel stimulable phosphor, a processfor the preparation of the same, a radiation image recording andreproducing method utilizing the same, and a radiation image storagepanel employing the same. More particularly, the invention relates to abismuth activated alkali metal halide stimulable phosphor.

2. Description of the Prior Art

For obtaining a radiation image, there has been conventionally employeda radiography utilizing a combination of a radiographic film having anemulsion layer containing a photosensitive silver salt and anintensifying screen.

As a method replacing the above-described radiography, a radiation imagerecording and reproducing method utilizing a stimulable phosphor asdescribed, for example, in U.S. Pat. No. 4,239,968 has been recentlypaid much attention. The method involves the steps of causing astimulable phosphor to absorb a radiation having passed through anobject or having radiated from an object; sequentially exciting (orscanning) the phosphor with an electromagnetic wave such as visiblelight or infrared rays (stimulating rays) to release the radiationenergy stored in the phosphor as light emission (stimulated emission);photoelectrically detecting the emitted light to obtain electricsignals; and reproducing the radiation image of the object as a visibleimage from the electric signals.

In the radiation image recording and reproducing method, a radiationimage is obtainable with a sufficient amount of information by applyinga radiation to the object at a considerably smaller dose, as comparedwith the conventional radiography. Accordingly, this method is of greatvalue, especially when the method is used for medical diagnosis.

As a stimulable phosphor employable in the above-described method, U.S.Pat. No. 4,239,968 discloses a rare earth element activated alkalineearth metal fluorohalide phosphor having the formula:

    (Ba.sub.1-x,M.sup.2+.sub.x)FX:yA

in which M²⁺ is at least one alkaline earth metal selected from thegroup consisting of Mg, Ca, Sr. Zn and Cd; X is at least one halogenselected from the group consisting of Cl, Br and I; A is at least oneelement selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr,Ho, Nd, Yb and Er; and x and y are numbers satisfying the conditions of0≦x≦0.6 and 0≦y≦0.2, respectively.

The phosphor gives emission (stimulated emission) in the nearultraviolet region when existed to an electromagnetic wave such asvisible light or infrared rays after exposure to a radiation such asX-rays.

The above-mentioned rare earth element activated alkaline earth metalhalide phosphor has been previsously known as a stimulable phosphoremployable in the radiation image recording and reproducing methodutilizing a stimulability thereof as described above, but almost nostimulable phosphor other than said phosphor has been known.

As a phosphor having the same alkali metal halide as host component asin the phosphor of the present invention, there has been previouslyknown a thallium or sodium activated cesium iodide phosphor (CsI:Tl orCsI:Na). This phosphor gives emission (spontaneous emission) whenexposed to a radiation such as X-rays, cathode rays and ultravioletrays.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a novel stimulablephosphor which is employable in the radiation image recording method,and a process for the preparation of the same.

Another object of the invention is to provide a radiation imagerecording and reproducing method utilizing the novel stimulable phosphorand a radiation image storage panel using said phosphor.

As a result of earnest studies, the present inventors have discoveredthat a specific bismuth activated alkali metal halide phosphor givessatisfactory stimulated emission

There is provided by the present invention a bismuth activated alkalimetal halide phosphor having the formula (I):

    M.sup.I X:xBi                                              (I)

in which M^(I) is at least one alkali metal selected from the groupconsisting of Rb and Cs; X is at least one halogen selected from thegroup consisting of Cl, Br and I; and x is a number satisfying thecondition of 0<x≦0.2.

The phosphor having the formula (I) of the invention can be prepared bythe processing comprising:

mixing starting materials for the phosphor in a stoichiometric ratiocorresponding to the formula (I):

    M.sup.I X:xBi                                              (I)

in which M^(I), X and x have the same meanings as defined above; and

firing the obtained mixture at a temperature within the range of500°-1000° C.

The bismuth activated alkali metal halide phosphor having the formula(I) gives stimulated emission in the near ultraviolet to blue regionwhen excited with an electromagnetic wave having a wavelength within therange of 450-900 nm after exposure to a radiation such as X-rays,ultraviolet rays and cathode rays. Particularly, the phosphor having theformula (I) in which M^(I) is Cs gives stimulated emission of highluminance.

The bismuth activated alkali metal halide phosphor having the formula(I) of the invention also gives spontaneous emission in the nearultraviolet to blue region when exposed to a radiation such as X-rays,ultraviolet rays and cathode rays.

The bismuth activated alkali metal halide phosphor having the formula(I) is employable in a radiation image recording and reproducing methodwhich comprises the steps of:

(i) causing the bismuth activated alkali metal halide phosphor havingthe formula (I) to absorb a radiation having passed through an object orhaving radiated from an object;

(ii) exciting said stimulable phosphor with an electromagnetic wavehaving a wavelength within the range of 450-900 nm to release theradiation energy stored therein as light emission; and

(iii) detecting the emitted light.

Especially, a phosphor having the formula (I) in which M^(I) is Cs showsprominently high sensitivity in the use for the radiation imagerecording and reproducing method.

In performing the above-described radiation image recording andreproducing method, the bismuth activated alkali metal halide phosphorhaving the formula (I) is advantageously employed in the form of aradiation image storage panel. The radiation image storage panelcomprises a support and at least one stimulable phosphor layer providedthereon which comprises a binder and a stimulable phosphor dispersedtherein, in which at least one phosphor layer contains the bismuthactivated alkali metal halide phosphor having the formula (I).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the stimulation spectra of CsCl:0.001Bi phosphor,CsBr:0.001Bi phosphor and CsI:0.001Bi phosphor (Curves 1, 2 and 3,respectively), which are examples of the bismuth activated alkali metalhalide phosphor according to the invention.

FIG. 2 shows stimulated emission spectra of CsCl:0.001Bi phosphor,CsBr:0.001Bi phosphor and CsI:0.001Bi phosphor (Curves 1, 2 and 3,respectively), which are examples of the bismuth activated alkali metalhalide phosphor according to the invention.

FIG. 3 is a schematic videw showing the radiation image recording andreproducing method according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The bismuth activated alkali metal halide phosphor of the presentinvention can be prepared, for instance, by the process described below.

As starting materials, the following materials can be employed:

(1) at least one alkali metal halide selected from the group consistingof RbCl, CsCl, RbBr, CsBr, RbI and CsI; and

(2) at least one compound selected from the group consisting of bismuthcompounds such as bismuth halide, bismuth oxide, bismuth nitrate andbismuth sulfate.

Further, ammonium halide (NH₄ X", in which X" is any one of Cl, Br andI) may be employed as a flux.

In the process for the preparation of the phosphor of the invention, theabove-mentioned alkali metal halide (1) and bismuth compound (2) are, inthe first place, mixed in the stoichiometric ratio corresponding to theformula (I):

    M.sup.I X:xBi                                              (I)

in which M^(I), X and x have the same meanings as defined above.

In the preparation of the phosphor of the invention, mainly from theviewpoint of enhancement in the luminance of stimulated emission, M^(I)in the formula (I) which indicates alkali metal is preferably Cs, andthe number for x which indicates the amount of bismuth activator ispreferably within the range of 5×10⁻⁴ ≦x≦10⁻².

The mixture of starting materials for the phosphor is prepared by anyone of the following procedures;

(i) simply mixing the starting materials (1) and (2); and

(ii) mixing the starting materials (1) and (2) in the form of asolution, and then drying the solution under reduced pressure, undervacuum or by spray drying under heating (preferably, 50°-200° C.).

The mixing is carried out using a conventional mixing apparatus such asa variety of mixers, a V-type blender, a ball mill and a rod mill in anycase of the above-described procedures (i) and (ii).

Then, the resulting mixture of the starting materials is placed in aheat-resistant container such as a quartz boat, an alumina crucible or aquartz crucible, and fired in an electric furnace. The temperature forthe firing suitably ranges from 500° to 1,000° C., and preferably rangesfrom 600° to 800° C. The firing period is determined depending upon theamount of the mixture of starting materials, the firing temperature,etc., and suitably ranges from 0.5 to 6 hours. As the firing atmosphere,there can be employed a weak reducing atmosphere such as a nitrogen gasatmosphere containing a small amount of hydrogen gas or a carbon dioxidegas atmosphere containing carbon monoxide gas; an inert gas atmospheresuch as a nitrogen gas atmosphere or an argon gas atmosphere; or anoxidizing atmosphere such as an air.

Through the firing procedure, a powdery phosphor of the presentinvention is produced. The powdery phosphor thus obtained may beprocessed in the conventional manner involving a variety of proceduresfor the preparation of phosphors such as a washing procedure, a dryingprocedure and a sieving procedure.

The phosphor of the invention prepared in accordance with theabove-described process is a bismuth activated alkali metal halidephosphor having the formula (I):

    M.sup.I X:xBi                                              (I)

in which M^(I) is at least one alkali metal selected from the groupconsisting of Rb and Cs; X is at least one halogen selected from thegroup consisting of Cl, Br and I; and x is a number satisfying thecondition of 0<x≦0.2.

The bismuth activated alkali metal halide phosphor of the inventiongives simulated emission in the near ultraviolet to blue region whenexcited with an electromagnetic wave having a wavelength within therange of 450-900 nm such as visible light or infrared rays afterexposure to a radiation such as X-rays, ultraviolet rays and cathoderays.

FIG. 1 shows stimulation spectra of CsCl:Bi phosphor, CsBr:Bi phosphorand CsI:Bi phosphor (Curves 1, 2 and 3, respectively), which areexamples of the bismuth activated alkali metal halide phosphor accordingto the invention.

As is evident from FIG. 1, the phosphors of the invention givestimulated emission upon excitation with an electromagnetic wave in thewavelength region of 450-900 nm. As is also evident from FIG. 1, peaksof the stimulation spectra of the phosphors according to the inventionare positioned in the longer wavelength side depending upon X of CsXwhich is host component of the phosphor in such an order of X as Cl(Curve 1), Br (Curve 2) and I (Curve 3). Particularly, the phosphor inwhich X is I is efficiently excited with infrared rays such as asemiconductor laser beam. Based on this fact, the wavelength region ofthe electromagnetic wave employed as stimulating rays, namely 450-900nm, has been decided for adoption in the radiation image recording andreproducing method of the present invention.

FIG. 2 shows stimulated emission spectra of the above-mentioned CsCl:Biphosphor, CsBr:Bi phosphor and CsI:Bi phosphor (Curves 1, 2 and 3,respectively), which are examples of the bismuth activated alkali metalhalide phosphor according to the invention.

As is evident from FIG. 2, the phosphors according to the invention givestimulated emission in the near ultraviolet to blue region, and eachpeak wavelength of the emission spectra is within the range of approx.350-450 nm. Accordingly, when the phosphor of the invention is excitedwith an electromagnetic wave having a wavelength within the range of500-850 nm after exposure to a radiation, it is easy to separate thestimulated emission from the stimulating rays, and the stimulatedemission shows high intensity. As is also evident from FIG. 2, peaks ofthe stimulated emission spectra of the phosphors of the invention arepositioned in the longer wavelength side depending upon X of CsX whichconstitutes the phosphor in such an order of X as Cl (Curve 1), Br(Curve 2) and I (Curve 3) in the same manner as the peaks of theabove-described stimulation spectra.

The stimulated emission spectra and stimulation spectra of the bismuthactivated alkali metal phosphors according to the present invention aredescribed above with reference to FIGS. 1 and 2, for the specificphosphors. It has been further confirmed that other phosphors accordingto the invention show similar stimulation spectra to those of theabove-mentioned specific phosphors, and further confirmed that they givesimilar stimulated emission spectra to those of the above-mentionedspecific phosphors, that is, stimulated emission spectra in the nearultraviolet to blue region, when excited with an electromagnetic wavehaving a wavelength within the range of 450-900 nm after exposure to aradiation, and that each peak wavelength of the stimulated emissionspeectra is within the range of approx. 350-450 nm.

The bismuth activated alkali metal halide phosphor of the presentinvention further gives emission (spontaneous emission) in the nearultraviolet to blue region upon excitation with a radiation such asX-rays, ultraviolet rays and cathode rays, and the spontaneous emissionspectrum thereof is almost the same as the stimulated emission spectrumthereof.

Since the wavelength region of the stimulation spectrum of the bismuthactivated alkali metal phosphor of the invention is so wide as 450-900nm, it is possible to optionally vary the wavelength of stimulating raysfor exciting the phosphor in the radiation image recording andreproducing method of the invention. It means that a source ofstimulating rays can be appropriately selected according to the purpose.For example, a semiconductor laser (having a wavelength in the infraredregion) which is in a small size and needs only weak driving power canbe employed as source of stimulating rays, and accordingly the systemfor carrying out the method can be made compact. Particularly when thephosphor having iodine as halogen which constitutes host component isemployed in the method, the phosphor can be efficiently excited using asemiconductor laser as a source of stimulating rays. From the viewpointsof the luminance of stimulated emission and of the separation onwavelength between the emitted light and stimulating rays, thestimulating rays are preferred to be an electromagnetic wave having awavelength within the range of 500-850 nm.

From the viewpoint of emission properties described hereinbefore, thephosphor of the invention is very useful as a phosphor for a radiationimage storage panel employed in the radiation image recording andreproducing method, or for a radiographic intensifying screen employedin the conventional radiography, both panel and screen being used inmedical radiography such as X-ray photography for medical diagnosis andindustrial radiography for non-destructive inspection.

The bismuth activated alkali metal halide phosphor having the formula(I) is preferably employed in the form of a radiation image storagepanel (also referred to as a stimulable phosphor sheet) in the radiationimage recording and reproducing method of the invention.

The radiation image storage panel basically comprises a support and atleast one phosphor layer provided on a surface of the support. Thephosphor layer comprises a binder and stimulable phosphor dispersedtherein. Further, a transparent protective film is generally provided onthe free surface of the phosphor layer (surface not facing the support)to keep the phosphor layer from chemical deterioration or physicaldamage.

The radiation image recording and reproducing method of the invention isdesired to be performed employing the radiation image storage panelcomprising a phosphor layer which contains the bismuth activated alkalimetal halide phosphor having the formula (I).

In the radiation image recording and reproducing method employing thestimulable phosphor having the formula (I) in the form of a radiationimage storage panel, a radiation having passed through an object orradiated from an object is absorbed by the phosphor layer of the panelto form a radiation image as a radiation energy-stored image on thepanel. The panel is then irradiated (e.g., scajnned) with anelectromagnetic wave in the wavelength region of 450-900 nm to releasethe stored image as stimulated emission. The emitted light isphotoelectrically detected to obtain electric signals so that theradiation image of the object can be reproduced as a visible image fromthe obtained electric signals.

The radiation image recording and reproducing method of the presentinvention will be described more in detail with respect to an example ofa radiation image storage panel containing the stimulable phosphorhaving the formula (I), by referring to a schematic view shown in FIG.3.

In FIG. 3 which shows a total system of the radiation image recordingand reproducing method of the invention, a radiation generating device11 such as an X-ray source provides a radiation for irradiating anobject 12 therewith; a radiation image storage panel 13 containing thestimulable phosphor having the formula (I) absorbs and stores theradiation having passed through the object 12; a source of stimulatingrays 14 provides an electromagnetic wave for releasing the radiationenergy stored in the panel 13 as light emission; a photosensor 15 suchas a photomultiplier faces the panel 13 for detecting the light emittedby the panel 13 and converting it to electric signals; an imagereproducing device 16 is connected with the photosensor 15 to reproducea radiation image from the electric signals detected by the photosensor15; a display device 17 is connected with the reproducing device 16 todisplay the reproduced image in the form of a visible image on a CRT orthe like; and a filter 18 is disposed in front of the photosensor 15 tocut off the stimulating rays reflected by the panel 13 and allow onlythe light emitted by the panel 13 to pass through.

FIG. 3 illustrates an example of the system according to the method ofthe invention employable for obtaining a radiation-transmission image ofan object. However, in the case that the object 12 itself emits aradiation, it is unnecessary to install the above-mentioned radiationgenerating device 11. Further, the photosensor 15 to the display device17 in the system can be replaced with other appropriate devices whichcan reproduce a radiation image having the information of the object 12from the light emitted by the panel 13.

Referring to FIG. 3, when the object 12 is exposed to a radiation suchas X-rays provided by the radiation generating device 11, the radiationpasses through the object 12 in proportion to the radiationtransmittance of each portion of the object. The radiation having passedthrough the object 12 impinges upon the radiation image storage panel13, and is absorbed by the phosphor layer of the panel 13. Thus, aradiation energy-stored image (a kind of latent image) corresponding tothe radiation-transmission image of the object 12 is formed on the panel13.

Thereafter, when the radiation image storage panel 13 is irradiated withan electromagnetic wave having the wavelength within the range of450-900 nm, which is provided by the source of stimulating rays 14, theradiation energy-stored image formed on the panel 13 is released aslight emission. The intensity of so released light is in proportion tothe intensity of the radiation energy which has been absorbed by thephosphor layer of the panel 13. The light signals corresponding to theintensity of the emitted light are converted to electric signals bymeans of the photosensor 15, the electric signals are reproduced as animage in the image reproducing device 16, and the reproduced image isdisplayed on the display device 17.

The operation of reading out the image information stored in theradiation image storage panel is generally performed by sequentiallyscanning the panel with a laser beam and detecting the light emittedunder the scanning with a photosensor such as photomultiplier through anappropriate light guiding means to obtain electric signals. In order toobtain a well-readable visible image, the read-out operation maycomprise a preliminary read-out operation and a final read-outoperation, in which the panel is twice irradiated with stimulating raysthough the energy of the stimulating rays in the former is lower thanthat in the latter (see: U.S. patent application Ser. No. 434,886). Theread-out condition in the final read-out operation can be suitablychosen based on the result obtained by the preliminary read-outoperation.

As the photosensors, solid-state photoelectric conversion devices suchas a photoconductor and a photodiode can be also used (see: U.S. patentapplication Ser. No. 610,582, Japense Patent Applications No.58(1983)-219313 and No. 58(1983)-219314, and Japanese Patent ProvisionalPublication No. 58(1983)-121874). For example, the photosensor isdivided into a great number of pixels, which may be combined with aradiation image storage panel or positioned in the vicinity of thepanel. Otherwise, the photosensor may be a linesensor in which pluralpixels are linearly connected or may be such one that corresponds to onepixel.

In the above-mentioned cases, there may be employed as the source ofstimulating rays, a linear light source such as an array in which lightemitting diodes (LED), semiconductor lasers or the like are linearlyarranged, in addition to a point light source such as a laser. Theread-out using such photosensor can prevent loss of the light emitted bya panel and can bring about the enhancement of S/N ratio of the image,because the photosensor can receive the emitted light with a largeangle. It is also possible to enhance the read-out speed, becauseelectric signals are sequentially obtained not by scanning the panelwith stimulating rays, but by electrical processing of the photosensor.

After reading out the image information stored in a radiation imagestorage panel, the panel is preferably subjected to a procedure oferasing the radiation energy remaining therein, that is, to the exposureto light having a wavelength in that wavelength region of stimulatingrays for the phosphor contained therein or to heating (see: U.S. Pat.No. 4,400,619 and Japanese Patent Provisional Publication No.56(1981)-12599). The erasing procedure can prevent the occurrence ofnoise originating from the after image in the next use of the panel.Further, the panel can be more effectively prevented from the occurrenceof noise attributable to natural radiations by carrying out the erasingprocedure twice, namely after the read-out and just before the next use(see: U.S. patent application Ser. No. 338,734).

In the radiation image recording and reproducing method of the presentinvention, there is no specific limitation on the radiation employablefor exposure of an object to obtain a radiation transmittance imagethereof, as far as the above-described phosphor gives stimulatedemission upon excitation with the electromagnetic wave after exposure tothe radiation. Examples of the radiation employable in the inventioninclude those generally known, such as X-rays, cathode rays andultraviolet rays. Likewise, there is no specific limitation on theradiation radiating from an object for obtaining a radiation imagethereof, so far as the radiation can be absorbed by the above-describedphosphor to serve as an energy source for producing the stimulatedemission. Examples of the radiation include γ-rays, α-rays and β-rays.

As the source of stimulating rays for exciting the phosphor which hasabsorbed the radiation having passed through or radiated from theobject, there can be employed, for instance, light sources providinglight having a band spectrum distribution in the wavelength region of450-900 nm; and light sources providing light having a single wavelengthor more in said region such as an Ar ion laser, a He-Ne laser, a rubylaser, a semiconductor laser, a glass laser, a YAG laser, a Kr laser, adye laser and a light emitting diode (LED). Among these sources ofstimulating rays, the lasers are preferred because the radiation imagestorage panel is exposed thereto with a high energy density per unitarea. Particularly preferred is the He-Ne laser from the viewpoints ofthe stability and output power thereof. The semiconductor laser is alsopreferred, because its size is small, it can be driven by a weakelectric power and its output power can be easily stabilized owing tothe direct modulation thereof.

The semiconductor laser is particularly preferably employed as a sourceof stimulating rays for exciting the phosphor having iodine(I) ashalogen which constitutes the host component, because the phosphor canbe efficiently excited therewith as described hereinbefore.

The radiation image storage panel employable in the radiation imagerecording and reproducing method of the invention will be described.

The radiation image storage panel, as stated hereinbefore, comprises asupport and at least one phosphor layer provided thereon which comprisesa binder and the above-described bismuth activated alkali metal halidephosphor having the formula (I) dispersed therein.

The radiation image storage panel having such structure can be prepared,for instance, in the manner described below.

Examples of the binder to be employed in the phosphor layer include:natural polymers such as proteins (e.g. gelatin), polysaccharides (e.g.dextran) and gum arabic; and synthetic polymers such as polyvinylbutyral, polyvinyl acetate, nitrocellulose, ethylcellulose, vinylidenechloride-vinyl chloride polymer, polyalkyl (meth)acrylate, vinylchloride-vinyl acetate copolymer, polyurethane, cellulose acetatebutyrate, polyvinyl alcohol, and linear polyester. Particularlypreferred are nitrocellulose, linear polyester, polyalkyl(meth)acrylate, a mixture of nitrocellulose and linear polyester, and amixture of nitrocellulose and polyalkyl (meth)acrylate.

The phosphor layer can be formed on a support, for instance, by thefollowing procedure.

In the first place, the stimulable phosphor particles and a binder areadded to an appropriate solvent, and then they are mixed to prepare acoating dispersion of the phosphor particles in the binder solution.

Examples of the solvent employable in the preparation of the coatingdispersion include lower alcohols such as methanol, ethanol, n-propanoland n-butanol; chlorinated hydrocarbons such as methylene chloride andethylene chloride; ketones such as acetone, methyl ethyl ketone andmethyl isobutyl ketone; esters of lower alcohols with lower aliphaticacids such as methyl acetate, ethyl acetate and butyl acetate; etherssuch as dioxane, ethylene glycol monoethylether and ethylene glycolmonomethylether; and mixtures of the above-mentioned compounds.

The ratio between the binder and the phosphor in the coating dispersionmay be determined according to the characteristics of the aimedradiation image storage panel and the nature of the phosphor employed.Generally, the ratio therebetween is within the range of from 1:1 to1:100 (binder:phosphor, by weight), preferably from 1:8 to 1:40.

The coating dispersion may contain a dispersing agent to assistdispersibility of the phosphor particles therein, and also contain avariety of additives such as a plasticizer for increasing the bondingbetween the binder and the phosphor particles in the phosphor layer.Examples of the dispersing agent include phthalic acid, stearic acid,caproic acid and a hydrophobic surface active agent. Examples of theplasticizer include phosphates such as triphenyl phosphate, tricresylphosphate and diphenyl phosphate; phthalates such as diethyl phthalateand dimethoxyethyl phthalate; glycolates such as ethylphthalyl ethylglycolate and butylphthalyl butyl glycolate; and polyesters ofpolyethylene glycols with aliphatic dicarboxylic acids such as polyesterof triethylene glycol with adipic acid and polyester of diethyleneglycol with succinic acid.

The coating dispersion containing the phosphor particles and the binderprepared as described above is applied evenly to the surface of asupport to form a layer of the coating dispersion. The coating procedurecan be carried out by a conventional method such as a method using adoctor blade, a roll coater or a knife coater.

A support material employed in the present invention can be selectedfrom those employed in the conventional radiographic intensifyingscreens or those employed in the known radiation image storage panels.Examples of the support material include plastic films such as films ofcellulose acetate, polyester, polyethylene terephthalate, polyamide,polyimide, triacetate and polycarbonate; metal sheets such as aluminumfoil and aluminum alloy foil; ordinary papers; baryta paper;resin-coated papers; pigment papers containing titanium dioxide or thelike; and papers sized with polyvinyl alcohol or the like. From theviewpoint of characteristics of a radiation image storage panel as aninformation recording material, a plastic film is preferably employed asthe support material of the invention. The plastic film may contain alight-absorbing material such as carbon black, or may contain alight-reflecting material such as titanium dioxide. The former isappropriate for preparing a high-sharpness type radiation image storagepanel, while the latter is appropriate for preparing a high-sensitivetype radiation image storage panel.

In the preparation of a known radiation image storage panel, one of moreadditional layers are occasionally provided between the support and thephosphor layer, so as to enhance the adhesion between the support andthe phosphor layer, or to improve the sensitivity of the panel or thequality of an image (sharpness and graininess) provided thereby. Forinstance, a subbing layer or an adhesive layer may be provided bycoating a polymer material such as gelatin over the surface of thesupport on the phosphor layer side. Alternatively, a light-reflectinglyaer or a light-absorbing layer may be provided by forming a polymermaterial layer containing a light-reflecting material such as titaniumdioxide or a light-absorbing material such as carbon black. In theinvention, one or more of these additional layers may be provided andthe constitution thereof can be selected according to the purpose of theradiation image storage panel.

As described in U.S. patent application Ser. No. 496,278 or EuropeanPatent Publication No. 92241, the phosphor layer-side surface of thesupport (or the surface of an adhesive layer, light-reflecting layer, orlight-absorbing layer in the case that such layers are provided on thephosphor layer) may be provided with protruded and depressed portionsfor enhancement of the sharpness of radiation image.

After applying the coating dispersion to the support as described above,the coating dispersion is then heated slowly to dryness so as tocomplete the formation of a phophor layer. The thickness of the phosphorlayer varies depending upon the characteristics of the aimed radiationimage storage panel, the nature of the phosphor, the ratio between thebinder and the phosphor, etc. Generally, the thickness of the phosphorlayer is within the range of from 20 μm to 1 mm, preferably from 50 to500 μm.

The phosphor layer can be provided on the support by the methods otherthan that given in the above. For instance, the phosphor layer isinitially prepared on a sheet (false support) such as a glass plate,metal plate or plastic sheet using the aforementioned coating dispersionand then thus prepared phosphor layer is overlaid on the genuine supportby pressing or using an adhesive agent.

The phosphor layer placed on the support can be in the form of a singlelayer or in the form of plural (two or more) layers. When the pluralphosphor layers are placed on the support, at least one layer containsthe aforementioned bismuth activated alkali metal halide phosphor havingthe formula (I), and the plural layers may be placed in such a mannerthat a layer nearer to the surface shows stimulated emission of higherintensity. In any case, that is, in either the single phosphor layer orplural phosphor layers, a variety of known stimulable phosphors areemployable in combination with the above-mentioned stimulable phosphor.

Examples of the stimulable phosphor employable in combination with thestimulable phosphor of the invention include the aforementioned phosphorand the phosphors described below;

ZnS:Cu,Pb, BaO.xAl₂ O₃ :Eu, in which x is a number satisfying thecondition of 0.8≦x≦10, and M^(II) O.xSiO₂ :A, in which M^(II) is atleast one divalent metal selected from the group consisting of Mg, Ca,Sr, Zn, Cd and Ba, A is at least one element selected from the groupconsisting of Ce, Tb, Eu, Tm, Pb, Tl, Bi and Mn, and x is a numbersatisfying the condition of 0.5≦x≦2.5, as described in U.S. Pat. No.4,326,078;

(Ba_(1-x-y),Mg_(x),Ca_(y))FX:aEu²⁺, in which X is at least one elementselected from the group consisting of Cl and Br, x and y are numberssatisfying the conditions of 0<x+y≦0.6, and xy=0, and a is a numbersatisfying the condition of 10⁻⁶ ≦a≦5×10⁻², as described in JapanesePatent Provisional Publication No. 55(1980)-12143;

LnOX:xA, in which Ln is at least one element selected from the groupconsisting of La, Y, Gd and Lu, X is at least one element selected fromthe group consisting of Cl and Br, A is at least one element selectedfrom the group consisting of Ce and Tb, and x is a number satisfying thecondition of 0<x<0.1, as described in the above-mentioned U.S. Pat. No.4,236,078; and

M^(II) X₂.aM^(II) X'₂ :xEu²⁺, in which M^(II) is at least one alkalineearth metal selected from the group consisting of Ba, Sr and Ca; each ofX and X' is at least one halogen selected from the group consisting ofCl, Br and I, and X=X'; and a and x are numbers satisfying theconditions of 0.1≦a≦10.0 and 0<x≦0.2, respectively, as described inJapanese Patent Provisional Publication No. 58(1983)-193162.

The radiation image storage panel generally has a transparent film on afree surface of a phosphor layer to physically and chemically protectthe phosphor layer. In the panel of the invention, it is preferable toprovide a transparent film for the same purpose.

The transparent film can be provided on the phosphor layer by coatingthe surface of the phosphor layer with a solution of a transparentpolymer such as a cellulose derivative (e.g. cellulose acetate ornitrocellulose), or a synthetic polymer (e.g. polymethyl methacrylate,polyvinyl butyral, polyvinyl formal, polycarbonate, polyvinyl acetate,or vinyl chloride-vinyl acetate copolymer), and drying the coatedsolution. Alternatively, the transparent film can be provided on thephosphor layer by beforehand preparing it from a polymer such aspolyethylene terephthalate, polyethylene, polyvinylidene chloride orpolyamide, followed by placing and fixing it onto the phosphor layerwith an appropriate adhesive agent. The transparent protective filmpreferably has a thickness within the range of approximately 0.1 to 20μm.

The present invention will be illustrated by the following examples, butthese examples by no means restrict the invention.

EXAMPLE 1

186.4 g. of cesium chloride (CsCl) and 0.266 g. of bismuth fluoride(BiF₃) were well mixed in a ball mill, to obtain a mixture of startingmaterials for the preparation of a phosphor.

The mixture thus obtained was placed in an alumina crucible, which was,in turn, placed in a high-temperature electric furnace. The mixtutre wasthen fired at 600° C. for 2 hours. After the firing was complete, thecrucible was taken out of the furnace and allowed to stand for cooling.Thus, a powdery bismuth activated cesium chloride (CsCl:0.001Bi)phosphor was obtained.

EXAMPLE 2

The procedure of Example 1 was repeated except for using 212.8 g. ofcesium bromide (CsBr) instead of cesium chloride, to obtain a powderybismuth activated cesium bromide (CsBr:0.001Bi) phosphor.

EXAMPLE 3

The procedure of Example 1 was repeated except for using 259.8 g. ofcesium iodide (CsI) instead of cesium chloride, to obtain a powderybismuth activated cesium iodide (CsI:0.001Bi) phosphor.

The phosphors prepared in Examples 1 to 3 were excited with a He-Nelaser (wavelength: 632.8 nm) after exposure to X-rays at 80 KVp, tomeasure stimulated emission spectra. The results are shown in FIG. 2.

In FIG. 2, Curves 1 to 3 correspond to the following spectra:

1: stimulated emission spectrum of CsCl:0.001Bi phosphor (Example 1);

2: stimulated emission spectrum of CsBr:0.001Bi phosphor (Example 2);and

3: stimulated emission spectrum of CsI:0.001Bi phosphor (Example 3).

The phosphors prepared in Examples 1 to 3 were excited with a lightwhose wavelength was varied in the range of 450-1,000 nm after exposureto X-rays at 80 KVp, to measure stimulation spectra at each peakwavelength of the stimulated emission thereof. The results are shown inFIG. 1.

In FIG. 1, Curves 1 to 3 correspond to the following spectra:

1: stimulation spectrum of CsCl:0.001Bi phosphor (Example 1);

2: stimulation spectrum of CsBr:0.001Bi phosphor (Example 2); and

3: stimulation spectrum of CsI:0.001Bi phosphor (Example 3).

Further, the phosphors prepared in Examples 1 to 3 were excited with theHe-Ne laser after exposure to X-rays at 80 KVp, to measure the intensityof stimulated emission. The intensity of stimulated emission wasmeasured by using a band pass filter (B-390; peak wavelength: 390 nm,half band width: 60 nm, transmissivity of peak wavelength: 78%) as afilter for receiving the stimulated emission. The results are set forthin Table 1.

In Table 1, the intensity of stimulated emission is expressed by arelative value based on the intensity of stimulated emission ofCsI:0.001Bi phosphor obtained in Example 3 being 100.

                  TABLE 1                                                         ______________________________________                                                    Relative Intensity of                                             Example     Stimulated Emission                                               ______________________________________                                        1           500                                                               2           700                                                               3           100                                                               ______________________________________                                    

EXAMPLE 4

Radiation image storage panels were prepared in the following mannerusing the three kinds of bismuth activated alkali metal halide phosphorsobtained in Examples 1 to 3.

To a mixture of the powdery phosphor and a linear polyester resin wereadded successively methyl ethyl ketone and nitrocellulose (nitrationdegree: 11.5%), to prepare a dispersion containing the phosphor and thebinder (10:1, by weight). Subsequently, tricresyl phosphate, n-butanoland methyl ethyl ketone were added to the dispersion. The mixture wassufficiently stirred by means of a propeller agitator to obtain ahomogeneous coating dispersion having a viscosity of 25-35 PS (at 25°C.).

The coating dispersion was applied to a polyethylene terephthalate sheetcontaining titanium dioxide (support, thickness: 250 μm) placedhorizontally on a glass plate. The application of the coating dispersionwas carried out using a doctor blade. The support having a layer of thecoating dispersion was then placed in an oven and heated at atemperature gradually rising from 25° to 100° C. Thus, a phosphor layerhaving a thickness of 250 μm was formed on the support.

On the phosphor layer was placed a transparent polyethylene terephthlatefilm (thickness: 12 μm; provided with a polyester adhesive layer on onesurface) to combine the transparent film and the phosphor layer with theadhesive layer.

Thus, a radiation image storage panel consisting essentially of asupport, a phosphor layer and a transparent protective film wasprepared.

The radiation image storage panels prepared in Example 4 were measuredon the sensitivity (i.e., intensity of stimulated emission) when excitedwith a He-Ne laser (wavelength: 632.8 nm) after exposure to X-rays at 80KVp. The measurement of the sensitivity thereof was done by using a bandpass filter (B-390; peak wavelength: 390 nm, half band width: 60 nm,transmissivity of peak wavelength: 78%) as a filter for receivingstimulated emission. The results are set forth in Table 2.

In Table 2, the sensitivity of the radiation image storage panels isexpressed by a relative value based on the sensitivity of the panelusing the CsI:0.001Bi phosphor obtained in Example 3 being 100.

                  TABLE 2                                                         ______________________________________                                                           Relative Sensitivity                                       ______________________________________                                        Panel using          500                                                      CsCl:0.001Bi phosphor (Example 1)                                             Panel using          700                                                      CsBr:0.001Bi phosphor (Example 2)                                             Panel using          100                                                      CsI:0.001Bi phosphor (Example 3)                                              ______________________________________                                    

We claim:
 1. A radiation image storage panel comprising a support and atleast one phosphor layer provided thereon which comprises a binder and astimulable phosphor dispersed therein, in which at least one phosphorlayer contains a bismuth activated alkali metal halide phosphor havingthe formula (I):

    CsX:xBi                                                    (I)

in which X is at least one halogen selected from the group consisting ofCl, Br and I; and x is a number satisfying the condition of 5×10⁻⁴≦x≦10⁻².
 2. The radiation image store panel as claimed in claim 1 whereX is Cl or Br.
 3. A bismuth activated alkali metal halide phosphorhaving the formula :

    CsX:xBi

in which X is at least one halogen selected from the group consisting ofCl, Br and I; and x is a number satisfying the condition of 5×10⁻⁴≦x≦10⁻².
 4. The phosphor as claimed in claim 3 where X is Cl or Br.